Comparison between Single Beam and Double Beam Atomic Absorption Spectrometer Systems

Comparison between Single Beam and Double Beam Atomic Absorption Spectrometer Systems

A spectrometer is one of the types of devices used for measuring electromagnetic radiation wavelengths, given they interact with a sample. A spectrophotometer, on the other hand, is a subcategory of spectrometer devices, which focus on the measurement of the light’s wavelength distribution. Therefore, it can be used for various purposes like UV – Visible, Infrared, Raman, Atomic Absorption Spectroscopy, etc.

Color detection is the primary use of a spectrophotometer where it takes a reference and provides the percentage of transmittance or reflectance from an object. Usually, a perfect Reflecting Diffuser is kept as a reference for this purpose. 

Here, we will be discussing the concept, along with its two major types: single beam spectrophotometer and double beam spectrophotometer.

Different Types of Spectrophotometers 

The Design of spectroscopic systems is based on the fundamental principle of light absorption by absorbing species-the Beer Lambert law. Over the years basic design has been based on single beam or double beam optics with the latter gaining prominence due to its distinct advantages.

Advances in electronics and detection systems have contributed further to the popularity of double beam systems. In the present article  the discussion will be limited  to Atomic Absorption Spectroscopy systems.

It is important to understand the optical layout of both single beam and double beam systems before you begin to appreciate the advantages of one over the other. So let’s get into the details and know these systems better. 

Understanding Single Beam Spectrophotometer

Single Beam Schematic Diagram
Single Beam Schematic Diagram

This spectrophotometer is used to measure the relative intensity of light before and after inserting a test sample. It functions by either blanking the instrument or standardizing it with respect to the reference. There are four basic single beam spectrophotometer components:

  • Monochromator
  • Detector
  • Light source (should be stable over time, low cost, wide wavelength range brightness, and long service life)
  • Sample (object)

You can understand more about it from its working.

Working: The light source comprising of a hollow cathode lamp emits sharp atomic line of the element whose determination is required. The light is modulated (switched on and off) rapidly by means of a rotating chopper located between the light source and the flame.

Modulation can also be achieved by pulsing the power (switched on and off rapidly) to the light source. Modulation serves to differentiate the light coming from the source lamp from the emission from the flame.

The modulated light is led to the flame where ground state atoms of the element of interest are present and after absorption is led to the monochromator which isolates the wavelength of interest which is then led to the detector.

Now that you know what a single beam spectrophotometer is and how it works, let’s see some of the advantages and disadvantages of this instrument.

Advantages of a Single Beam Spectrophotometer

  • Cost-Effective: Single beam instruments are less expensive as compared to the other alternative. 
  • Better Performance: High energy throughput due to the non-splitting of the source beam results in high sensitivity of detection.

Disadvantages of a Single Beam Spectrophotometer

  • Instability: This happens due to lack of compensation for disturbances like electronic circuit fluctuations, voltage fluctuations, mechanical component’s instability, or drift in the energy of light sources. Such drifts cause abnormal fluctuations in the results.

Understanding Double Beam Spectrophotometer

Double Beam AAS Schematic Diagram
Double Beam AAS Schematic Diagram

Unlike the single beam variant, this one does not standardize or blank the instrument before use. Instead, it splits the beam into two parts for the same purpose. One of the beam parts is passed through the given object while the other passes through a reference standard. The number of components also get increased in this spectrophotometer.

  • Sample Holder
  • Interpreter
  • Light Source
  • Detector
  • Monochromator

Let’s see the working of this spectrophotometer.

Working: The light beam from the source is split into sample beam and reference beam by the mechanical chopper. The reference beam monitors the lamp energy whereas the sample beam reflects sample absorption.

The observed absorbance measurement is the ratio of the sample and reference beams which are recombined before moving to the monochromator. This arrangement compensates the effects due to drift in lamp intensity, electronic and mechanical fluctuations which affect both the sample and reference beams equally.

Now let’s see what the advantages of using a double beam spectrophotometer are. 

Advantages of a Double Beam Spectrophotometer

  • More Reliable Detection: Modern improvements in optics permit a high level of automation and offer the same or even better level of detection as compared to earlier single beam systems. Instability factors due to lamp drift, stray light, voltage fluctuations do not affect the measurement in real-time.
  • No Warm-Up Time: Little or no lamp warm-up time is required. This not only improves the throughput of results but also conserves lamp life.

Single vs. Double Beam Spectrophotometer: Which One to Pick?

As per the advantages, we can say that double beam spectrophotometers would be more beneficial. But you must consider the following points before selecting one. 

  • Cost: Obviously, a more significant price will be associated with the more complex structured instrument. So you will have to add more funds into your spectrophotometer budget if you want to go for the more advanced option. However, you can consider spending a little extra because of all the benefits that the double beam alternative is offering. Plus, the overall cost gets reduced in it due to the conservation of lamp life. 
  • Hassle: If you want to keep your work faster, you need to go for the double beam spectrophotometer. It passes the light from reference and the sample at the same time. Therefore, you don’t have to go through any hassle. On the other hand, the single beam option requires you to put in the reference first, then standardize the instrument according to it, and then remove it to place in the sample finally. Opt for this alternative only if you have the energy and time to do all the excess work. 

Although both options work fine, a double beam spectrophotometer does have a few benefits over a single beam. 

Your choice majorly depends on your priorities. If you want to keep the price low and can work with little instability and hassle, you can go with a single UV beam spectrophotometer. But in case you want overall better performance, you will have to opt for the double beam alternative.  

The topic is open for discussion and you may offer your valued comments.

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  1. Dear Sir, I’ve background in Physics. Recently,my school bought an AAS and no one has been able to install it and I do not know anything about it but would like to know so as to be able to use it for analysis. Please,what do I do in other to become proficient in using the AAS for analysis?

    1. Dear Solomon,
      You can make a start by registering for the AAS free course . It will introduce you to the basic components and their role. Next you can enrol for the advanced level certificate course on AAS which is to be launched in near future.The content of this course will be interesting and the visuals will explain the operational aspects.I am sure it will meet your desired goal.
      In case you have any queries please feel to contact me on my e-mail as well.

      1. Dear Sir,

        I am an instrumentation Engg.,
        please give me the dis-advantages of Double beam AAS .

        with regards,

        1. Dear Shivananda,
          The disadvantage, if you may say so, is only reduction in sensitivity due to splitting of beam into two portions. These days due to advances in electronics and improvements in sensitivity of detection systems even this disadvantage has been overcome and today all manufacturers are providing double beam systems.

      2. Sir,
        We have purchased AAS. What is difference in oxy-acetylene and pure acetylene gas used in aas. Also can you tel me the application of different gases used in AAS

        1. The term oxy-acetylene refers to the flame. The gases used for sustaining combustion in this case are acetylene which is fuel gas and air which has presence of oxygen which acts as oxidant to support the flame ignition. Majority of elements can be studied with acetylene and air combinations.Pure oxygen is not used due to high flame velocity of acetylene-oxygen flame which can result in flashback and explosive bang in the burner chamber.The other common combination is acetylene and nitrous oxide.In this case the oxidant required to support combustion is nitrous oxide , This flame results in higher temperature than air -acetylene flame. Such temperatures are necessary to decompose metals forming stable oxides(refractory elements)

  2. Dear sir,
    I am planing to purchase a new spectrophotometer for my Biochemistry lab.
    I am working on capsanthin analysis.
    Kindly suggest , should I go with single beam or double beam is more beneficial.
    Pls replay.

    Thank you

  3. Dear Dr/

    I am working as an assistant professor in India and i am interested in buying a spectrophotometer for my lab. Which one do you think is the best for my lab. I have a budget of 8 lacs. My lab involves working with nucleic acids, enzymes, proteins. Also since bachelor students of biotechnology program will be also using this machine for their routine practicals so kindly keep this in mind before suggesting anything.

    Thank in advance. I would really appreciate your help.

    1. Dear Prof Singh,
      From your required applications details it seems that you should go for a UV- VIS Spectrophotometer instead of AAS.Please confirm the same so that we may suggest a suitable configuration.

    1. Dear John,
      Please let us have more details about your activities on my mail id so that we can respond accordingly.

  4. Hello Dr. I am a technologist in the chemistry laboratory of a polytechnic where I work. we have a DLC-20 HPLC instrument that has never been used because it has not been installed, an sadly I can not operate it!!! can I get a customized training on the above HPLC instrument in your facility? Please furnish me on how I can go about arranging such training. ?

    1. Hi Patrick,
      The best option would be to get it installed and the installation engineer should be in a position to familiarize you with its operation.

  5. We are planning to buy a UV-Vis spectrophotometer and we want to establish whether to have a single or double beam optics. We will be using this for P and Mo analysis in ferrous metals. Please advise..Thanks.

    1. Hi Gena,
      You should undoubtled go for double beam system as a combination of sophisticated modern day optics and electronics offers same sensitivity and interference free performance as single beam systems.

  6. How do I contact you one on one for AAS training especially with regard to geological science am a geology lab technologist.

  7. Dear friend ,
    I am a service engineer for analytical instrument . I have worked since 1989 .
    I work now for my own, istanbul , Turkey.
    I still to learn more and more. I just meet this website while look for chopper funtions in the instrument.
    Best Regarda

  8. I just want to say thank you for been responsive on this forum.

    Thanks Prof. for your contribution.
    I hope to be a Prof in years to come.

  9. Dr. D. Bhanot deserves a world of great appreciation for sharing his knowledge of instrumentation.We have made up our mind to purchase double beam over single beam.

    Thanks Dr. Sahib.

  10. the singel beam and duble beam structure are not correct. I got zero in my exam. The sample cell should be after the monochromator.

    1. In single beam instruments the light beam is passed through the sample without splitting whereas in double beam instruments the beam is split in two . One portion passes through the sample and the other through a reference blank. Double beam instruments give benefit of blank correction in real time.

  11. Wow! This could be one particular of the most helpful blogs We’ve ever arrive across on this subject.
    Basically Great. I’m also an expert in this topic so I
    can understand your hard work.

  12. Im planning my own double beam setup using a supercontinuum source. What instrument is used for the detection of the reference and sample beam

    1. Need further clarification on your question. Are you going to add a double beam AAS.The instrument has an arrangement of automatic comparison of reference and sample beams either using pulsing sequence of beams or by means of a mechanical chopper.

    1. Sample is the solution which contains the element whose concentration is required.Reference contains the same solvent minus the element whose concentration is required to be determined . The concept is similar to UV-Vis in which the the two solutions are kept in seperate cells inside the sample compartment.In AAS however,reference refers known standard concentration the calibration plot a standard solution of known concentration.The reference is aspirated in different concentrations to generate the calibration plot before aspiration of the sample solution.

  13. What are the causes of error in the optical density measurement in single beam absorption spectrophotometer? Describe how double beam absorption spectrophotometer is used to overcome these error?

    1. The different sources of errors are already covered in the article.In single beam instrument the common errors are electrical fluctuations during measurement,variations in blank and sample measurement conditions, etc. All such differences between blank and sample are taken care in double beam instruments in which case all measurements are made in real time.

  14. Hi sir ! what can you say about High resolution continuum spectrum atomic absorption spectrophotometer

        1. Thanks for giving the instrument make.Honestly I am not having clarity on your query. Please specify on what aspect you require my comment.

  15. Could double beams system be better than single beams in term of intensity stability?
    Could I use spare power source from single to double beam instrument

  16. sir can u plz tell me.. that in atomic spectroscopy either its emission or absorption..single beam spectrophotometer is used?? and in uv vis and ir both single and double beam spectrophotometerz can be used? thanks in advance

  17. The monochromator in above diagram is placed after the sample , which in actual should be placed before the sample.

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